Process of making cellular metal structures



T. L. SPINK 3,218,684

PROCESS OF MAKING CELLULAR METAL STRUCTURES Nov. 23, 1965 2 Sheets-Sheet 1 Filed Aug. 31, 1962 Fr 0m gas pressure SOL/l" 6 e From wafer From wa/er b. 0 o o g y INVENTOR. 7h omas' L S OIhk Nov. 23, 1965 T. L. SPINK 3,218,684

PROCESS OF MAKING CELLULAR METAL STRUCTURES Filed Aug. 31, 1962 2 Sheets-Sheet 2 V Ck p Ham 90-5 57 V0 pressure- From wo/er From wa/er Supp/3 ooood'b INVENTOR. Thomas L. \Sp/nk HGfNT United States Patent 3,218,684 PROCESS OF MG CELLULAR METAL STRUCTURES Thomas L. Spink, Midland, Mich, assignor to The Dow Chemical Company, Midland, Mich, a corporation of Delaware Filed Aug. 31, 1962, Ser. No. 222,253 4 tilaims. (Cl. 22-200) This invention relates to the leaching of solids from a porous metal structure and more particularly is concerned with a process for leaching water-soluble salts from cellular metal structures.

A useful process for preparing a metal structure having inter-connected cells is to pack a mold of the desired shape with spherical granules of predetermined size of a metal insoluble constituent having a melting point above that of the molten metal. The mold is filled with the granules such that these are in direct contact. The sopacked mold is preheated and the molten metal poured therein. The molten metal flows around the granules of the insoluble constituent and into the interstitial spaces between the granules thereby forming a cellular structure of metal surrounding the granules. After cooling the metal insoluble constituent is dissolved from the metal by use of a solvent relatively inert to the metal. A porous metal element or structure is thus obtained of a predetermined cellular structure having the shape of the mold and voids of the size of the granules with which the mold was packed.

This method has been found to be particularly suitable for use in preparing cellular magnesium and magnesium alloy structures which find a particular utility as a fuel component in rocked propellant systems. The terms magiesiurn, magnesium based metal and magnesium alloy as used herein are meant to include those metallic compositions containing at least about 80 weight percent magnesium. With these magnesium based metals which have a relatively low melting point, conveniently inorganic salts having a higher melting point than the metal can be used as mold packing. Illustrative examples of such suitable salts are sodium chloride, potassium chloride, magnesium chloride and the like. These readily are leached from the cast structure by water.

, Ordinarily the salt is removed from a cast, porous magnesium structure by (l) immersing the sample in a water bath, (2) immersing the compact in a bath having turbulence created by water being forced into the bath or (3) by impinging water under pressure against and within the structure. Salt removal is achieved by these techmques. However, in all of these methods an extended period of time is required to achieve substantially complete removal of the salt from the cast structure.

With the magnesium based metal structures when the metal is in contact with concentrated salt solutions for extended periods of time the solutions act corrosively on the magnesium thereby giving undesirably high metal loss. Additionally, as the corrosive reaction continues, the build-up of solid corrosion product within the cells many times results in a pressure suflicient to rupture the cells and even burst open the sample.

It is a principal object of the present invention therefore to provide a method for producing a porous metal structure by rapid leaching of particulate metal insoluble granules from within the structure.

It is another object of the present invention to provide a process for leaching water-soluble salt granules from within a magnesium based porous structure wherein detrimental corrosion of the magnesium during the leaching is substantially completely avoided.

It is an additional object of the present invention to provide a novel apparatus for leaching metal insoluble granules from a tubular cast metal structure having interconnected cells.

These and other objects and advantages will become apparent from the detailed description presented hereinafter when read in conjunction with the accompanying drawings.

In the present process, the cast structure is subjected to the action of a solvent for the metal insoluble granules under pressure while the structure simultaneously is being subjected to high speed rotation. The novel combination of treatment steps assures highly efiicient dissolution and removal of the solid granules from the structure without having an undesirably high concentration of a concentrated solution of the interstitial component in contact with the metal structure for any appreciable period of time.

The present process is illustrated further by the embodiment shown in the figures. This particular form of the apparatus has been shown to be highly useful for removing water-soluble salts from a substantially tubular-shaped magnesium cast structure.

As shown in FIGURE 1, the apparatus consists of an elongated tubular member, i.e. hollow shaft 11 having one blind end 12. This member has threads 13 extending upward from the blind end 12 and defines a plurality of small ports or openings 14 spaced around the wall near this end :12 of the shaft 11. These openings extend a predetermined distance up the shaft away from the end 12 such that they cover a distance of less than the length of a cast tubular magnesium structure 15 to be leached, but substantially extend up the shaft a distance about equal to the length of the structure. The threads 13 extend up the shaft 11 a distance beyond the uppermost of the openings 14.

The shaft 11 is equipped with a pulley 16 and connected through a belt 17 to a power source of rotary motion such as an electric motor 18.

The other end of the shaft 11 is connected through a valve 19 and flow meter 20 by means of a conduit 21 to a pressurized water supply not shown.

In a leaching operation, a lock nut 22 is threaded onto the shaft 11. A washer 23 is placed onto the shaft 11 abutting the nut 22 followed by a gasket 24. These three members are positioned on the shaft 11 such that they are above the ports 14 in shaft 11. The cast tubular magnesium structure 15 is placed onto the shaft 11 and has one of its ends positioned against the gasket 24. A second gasket 25, washer 26 and lock nut 27 are then put onto the blind end 12 of the shaft 11 of the assembly and nut 27 is tightened to provide a substantially liquid tight fit of the cast magnesium structure 15 against the two gaskets 24 and 25 in this holding assembly. A separate conduit 28 having a blind end 29 and defining a number of small ports 30 in one section of its wall is placed near the assembly holding the cast magnesium structure 15. This conduit 28 is of a predetermined design and position so that the ports 30 extend substantially over a distance equal to the length of the cast structure 15 and preferably are aligned in a row in the wall of the conduit 28 so that they directly face the cast structure 15.

In utilizing this embodiment to leach a water-soluble salt from within a cellular magnesium structure a metered supply of pressurized water is fed through the flow meter 20 and valve 19 through shaft 11 and out of ports 14 into the annular space 31 between the shaft 11 and the cast structure 15. The action of the flowing high pressure water dissolves the salt from the cast structure and forces it to the outside of the structure. Simultaneously pressurized water is fed through conduit 28. This water exits through ports 30 and serves to continuously rinse away 3 from the metal structure the concentrated salt solutions resulting from the leaching operation.

An alternative embodiment of the leaching apparatus is shown in the fragmentary view of FIGURE 2. In this embodiment, the actual leaching assembly is the same as shown for the apparatus in FIGURE 1. The modification involves changing the liquid input for directing the pressurized water into shaft 11. In this latter device, valve v19 is replaced by a three way valve 32. The third connection of the three way valve 32 connects to conduit 33 which contains a check valve 34 and is connected through a pressure pump 35, preferably of a controlled valve delivery type, to a fluid supply reservoir 36.

In the leaching process as carried out utilizing the embodiment of the apparatus shown in FIGURE 2, at a predetermined time near the end of the leaching operation small amounts of a leaching promoter, for example hydrochloric acid can be metered from reservoir 36 through the metering pump 35 into the water stream in shaft 11 into the leaching system, to provide a dilute hydrochloric acid solution. With magnesium based structures an acid solution containing from about 0.01 to about 0.03 percent hydrogen chloride has been found to be satisfactory. This additional step serves to clean from the metal surface any metal oxide surface film which may be formed during the leaching operation and provides a gloss-like metal surface.

Additionally, the leach water feed conduit 21 in either of the embodiments can be fitted with a valved inlet assembly 37 communicating with a pressurized inert gas source (not shown). This added feature provides a ready means of removing excess water remaining in the cellular structure after the leaching operation. In utilizing this latter device an inert high pressure gas, for example nitrogen, is fed into the system while the leached cellular struc; ture is being rotated. The combination of centrifugal force from the spinning body and high pressure gas serves to readily remove excess water from the interconnected cells of the leached metallic structure.

The rates of fluid flow and pressures to be employed are not critical except that these must be of a value so as not to burst the cells and deform the structure during the leaching operation. However, these must be of sufiicient force, i.e., of a sufficiently rapid flow rate and high pressure, that effective leaching is obtained.

Conviently, tap water as supplied by most household and commercial water systems has been found to be of satisfactory pressure and flow rate for the present leaching operation of a magnesium cellular structure. These systems ordinarily operate at pressures of from about 20 to about 60 pounds per square inch pressure gauge.

The speed of rotation of the cast structure also is not critical. However, this speed should be such that sutficient centrifugal force is present to force the water to the outside of the structure. Therefore, as is understood by one skilled in the art the amount of rotational energy to be supplied will be dependent somewhat on the size and mass of the structure being leached and the cell size of the structure. The actual rotational speeds to be employed ordinarily range from several hundred up to several thousand or more revolutions per minute. Lower speeds ordinarily are used for large, massive structures, higher speeds are employed for smaller, lightweight structures to obtain the desired effective centrifugal force whereby the leach solution is directed to the periphery of the structure.

The illustration presented hereinbefore for descriptive purposes has shown one apparatus and method for leaching soluble salts from magnesium. However, it is apparent to one skilled in the art that this same apparatus or other embodiments thereof and manipulative procedures can be used to leach any of a variety of metal soluble interstitial components from a given cellular metallic structure with a suitable solvent in accordance with the present novel process.

The present invention is illustrated further by the following example but is not meant to be limited thereto.

Example A cast tubular magnesium structure was prepared by filling a suitable mold with prilled sodium chloride pellets and pouring molten magnesium over the pellets. The resulting tubular structure was about 2 inches in diameter and had a wall thickness of about one inch and was about three inches high. This tubular member was placed into a leaching assembly having about a one inch diameter shaft and incorporating the embodiments shown in FIG- URE 2. The sample was rotated at a speed of about 2,000 revolutions per minute While simultaneously passing water at a pressure of about 40 to 50 pounds per square inch gauge and a rate of about gallons per hour through the sample. At the same time the exterior of the sample was rinsed by passing water at a maximum tap pressure through a perforated conduit and directing the streams of water issuing from the ports of conduit directly on the exterior of the rotating cast structure. The water temperature ranged from about 12 to about 17 C. The sample was leached with water for about 30 minutes. After this time hydrochloric acid was pumped into the metered leach water at a rate to provide a total of 0.017 percent hydrogen chloride in the leach water based on a flow rate of 90 gallons per hour. Leaching with this acid solution was continued for an additional 20 minutes after which time the sample was thoroughly flushed with water for an additional 10 minutes. Following this the flow of leach water was stopped and pressurized nitrogen was forced through the cellular magnesium member While still maintaining the rotation of this member to drive the excess leach water from the pores of the structure.

The resulting product had a glossy metallic appearance and it appeared to be free of detrimental oxide. X-ray analysis of hte leached product indicated the salt was substantially completely leached from the cells of the metal and there was substantially no density variation in the metal as might be expected from the use of a too strongly acidic acid solution.

Additionally aluminummagnesium alloys cast around potassium chloride granules and aluminum-silicon alloys containing from about 4 to 7 weight percent silicon cast around magnesium chloride can be satisfactorily leached by the present pressurized water-rotating cast structure method.

A porous beryllium structure can be obtained by leaching a beryllium structure having beryllium oxide granules as a metal-insoluble packing with an acid in accordance with the present process. Also, aluminum oxide employed as a filler for a porous titanium structure readily can be leached from the resulting cast structure with hot caustic solution in accordance with the present novel process.

Additionally, cellular zicronium cast structures and those of other metals and alloys can be freed of the metal-insoluble granular constituent by employing the instant novel leaching process.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that I limit myself only as defined in the appended claims.

I claim:

1. In a process for producing a porous magnesium based metal structure of a predetermined shape and having interconnected cells and formed by pouring a member selected from the group consisting of magnesium and magnesium base alloys into a mold containing granules of a water soluble inorganic salt having a melting point above that of the molten magnesium-base metal selected from the group consisting of sodium cholride, potassium chloride and magnesium chloride wherein said salt granules in said mold are in direct contact and solidifying the molten magnesium based metal, the improvement.

' which consists in the steps of:

(1) rotating the substantially solid cast structure of porous magnesium based metal having solid granules of said water soluble salt contained therein,

(2) simultaneously subjecting the rotating structure to the action of pressurized Water thereby to leach said salt granules from the interconnected pores of said magnesium structure, the entire stream of said pressurized water being caused to pass through at least a portion of the solid cast structure at a pressure greater than atmospheric,

(3) continually rinsing the outer surface of said rotating structure during the leaching operation with pressurized water thereby to wash away from the surface of said porous metal structure the concentrated aqueous solution of said salt resulting from said leaching action, and

(4) continuing the simultaneous structure rotating,

leaching and rinsing operation until said porous magnesium base structure is substantially free of said salt.

2. In a process for producing a porous magnesium tubular structure having interconnected cells and formed by pouring molten magnesium into a mold filled with granules of sodium chloride and solidifying the magnesium the improvement which consists in the steps of:

( 1) placing the substantially solid cast structure in a rotatable assembly, said assembly having a perforated, hollow shaft extending within the open center of said tubular structure and said rotatable assembly having members completely enclosing the ends of said solid cast structure in a water-tight manner,

(2) rotating said structure,

(3) directing pressurized water out of the perforations in said hollow shaft against the inner wall of said tubular magnesium structure While said structure is rotating thereby to dissolve said sodium chloride granules in the pores of said structure and force the resulting sodium chloride solution to the exterior of said cast structure,

(4) directing pressurized water against the outer surface of said rotating structure during the leaching operation thereby to wash said sodium cholride leach solu tion from the porous metal surface, and

(5) continuing the simultaneous structure rotation, water leaching and Water rinsing operation until said porous magnesium structure is substantially freed of said sodium chloride.

3. The process as defined in claim 2 and including the step of incorporating hydrochloric acid into the leach water during the final portion of the leach operation, the amount of said hydrochloric acid added providing an aqueous leach solution having from about 0.01 to about 0.03 percent hydrogen chloride.

'4. The process as defined in claim 2 and including the step of subjecting the leached porous metal structure to a high pressure inert gas stream while rotating said structure.

References Cited by the Examiner UNITED STATES PATENTS 1,436,809 11/1922 Junguera 23269 2,178,701 11/1939 Petre 134-33 2,315,394 3/1943 BroSius 22196 2,767,110 10/1956 Frekko 13433 XR FOREIGN PATENTS 25,702 1911 Great Britain.

OTHER REFERENCES Lightweight Cellular Metal by Polonski, Lipson and Markus, Published in Modern Casting, vol. 39, No. 2, February 1961, pages -79 relied on.

MARCUS U. LYONS, Primary Examiner.

SAMUEL FEINBERG, MICHAEL V. BRINDISI,

Examiners. 

1. IN A PROCESS FOR PRODUCING A POROUS MAGNESIUM BASED MEATL STRUCTURE OF A PREDETERMINED SHAPE AND HAVING INTERCONNECTED CELLS AND FORMED BY POURING A MEMBER SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM AND MAGNESIUM BASE ALLOYS INTO A MOLD CONTAINING GRANULES OF A WATER SOLUBLE INORGANIC SLAT HAVING A MELTING POINT ABOVE THAT OF THE MOLTEN MAGNESIUM-BASE METAL SELECTED FROM THE GROUP CONSISTING OF SODIUM CHLORIDE, POTASSIUM CHLORIDE AND MAGNESIUM CHLORIDE WHEREIN SAID SALT GRANULES IN SAID MOLD ARE IN DIRECT CONTACT AND SOLIDIFYING THE MOLTEN MAGNESIUM BASE METAL, THE IMPROVEMENT WHICH CONSISTS IN THE STEPS OF: (1) ROTATING THE SUBSTANTIALLY SOLID CAST STRUCTURE OF POROUS MAGNESIUM BASED METAL HAVING SOLID GRANULES OF SAID WATER SOLUBLE SALT CONTAINED THEREIN, (2) SIMULTANEOUSLY SUBJECTING THE ROTATING STRUCTURE TO THE ACTION OF PRESSURIZED WATER THEREBY TO LEACH SAID SALT GRANULES FROM THE INTERCONNECTED PORES OF SAID MAGNESIUM STRUCTURE, THE ENTIRE STREAM OF SAID PRESSURIZED WATER BEING CAUSED TO PASS THROUGH AT LEAST A PORTION OF THE SOLID CAST STRUCTURE AT A PRESSURE GREATER THAN ATMOSPHERIC, (3) CONTINUALLY RINSING THE OUTER SURFACE OF SAID ROTATING STRUCTURE DURING THE LEACHING OPERATION WITH PRESSURIZED WATER THEREBY TO WASH AWAY FROM THE SURFACE OF SAID POROUS METAL STRUCTURE THE CONCENTRATED AQUEOUS SOLUTION OF SAID SALT RESULTING FROM SAID LEACHING ACTION, AND (4) CONTINUING THE SIMULTANEOUS STRUCTURE ROTATING, LEACHING AND RINSING OPERATION UNTIL SAID POROUS MAGNESIUM BASE STRUCTURE IS SUBSTANTIALLY FREE OF SAID SALT. 